Finishing | Cleaners & Restorers | Causes of Discoloration
Decks Should Be Finished For Optimal Performance:
Although cedar is a naturally durable wood ideal for decks, its performance is
enhanced when protected by an appropriate finish. Decks have full exposure
to sun and rain, which greatly accelerates the weathering process. In fact, so
aggressive are the effects of weather extremes in some areas of the country
that a deck may need cleaning, restoring and refinishing as frequently as
every two to five years depending upon the finish used. Decks should never
be allowed to weather before finishing.The simplest, but most labor-intensive,
finish to maintain on a cedar deck is a water-repellent preservative, which may
have to be applied annually. The next easiest is a semi-transparent oil-based
stain. Both types of finishes are extremely effective in stopping the absorption
of water and are recommended. It is important to ensure that the product has
been specifically formulated to withstand the abrasive effects of foot traffic.
Oil and latex solid-color stains, (also called heavy-bodied or opaque stains),
paints, and other film-forming finishes are not recommended.
Semi-transparent stains are pigmented finishes. The pigments provide color
and some ultraviolet protection and greatly increase the durability of the finish.
Apply finishes the full length of only two or three boards at a time to avoid lap
marks. Do not apply more stain than the cedar will absorb because the excess
stain will appear as a shiny area on the surface. For extra protection against
mildew, an annual or even semi-annual application of a water-repellent
preservative formulated with a mildewcide may be effective.
If there is uncertainty over whether to use a water-repellent preservative or a
stain, first apply a water-repellent preservative. It is possible to switch to a
semi-transparent stain when the deck needs to be refinished. Even if the deck
has been maintained with a water-repellent preservative for many years, an
oil-based semi-transparent stain will perform satisfactorily.
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Decks Cleaners And Restorers:
Until about ten years ago there were few, if any, products of this type on the
market. Most coatings manufacturers recommended that decks and other
exposed wood surfaces be cleaned before refinishing. The usual
recommended cleaners were household products such as detergents for dirt
removal and liquid bleach for mildew removal.
Household cleaners and bleaches can be effective to some extent but they
have their limitations. Also, since they are not usually designed for deck
cleaning applications they can present some handling problems to
do-it-yourselfers and contractors. For example, liquid household bleach
should not be mixed or used directly with ammonia or any other detergents or
cleaners containing ammonia since the resulting chemical reaction can form a
potentially dangerous gas.
About tn years ago products began appearing in the market that were
specifically designer to clean and restore weathered wood surfaces such as
decks and siding. Today there are a variety of such products available. Deck
cleaners and restorers generally fall into one of there categories-- chlorine
bleaches, oxygen bleaches, or oxalic acid-based formulas. Each of these is
Common types of chlorine bleach used in deck cleaning products are sodium
hypochlorite, calcium hypochlorite and dichloroisocyanurate. The first two are
typically used in laundry detergents while the last is a swimming pool additive.
These chemicals are effective against mildew but do little to remove dirt or
other surface deposits (which is why bleach alone does not get clothes clean).
When used on wood decks chlorine-based bleached products can do more
harm than good. They can result in the wood's having a whitish unnatural tone
due to the bleaching of natural components or a fuzzing of the wood's surface
due to the loosening of small fibers during the cleaning process. Moreover, if
not rinsed properly, the chlorine salt residues can result in premature greying
of wood from the action of sunlight.
As noted previously, household bleach and other products containing
chlorine-based bleaches should not be mixed with products containing
Products in this category are usually based on disodium peroxydicarbonate,
commonly known as sodium percarbonate, an ingredient present in some
color safe fabric bleaches. Sodium percarbonate is a powder. When added to
water it forms hydrogen peroxide -- a common oxygen bleach -- and sodium
carbonate (soda ash). Hydrogen peroxide is commonly used as a disinfectant
and a stripper for hair coloring. On wood it is effective in removing mildew
stains and weathered grey residue from UV (sunlight) degradation. The
sodium carbonate acts as a built-in detergent, removing dirt and other
deposits. Thus, sodium percarbonate-based cleaners are effective in
removing dirt, mildew and weathered grey residues. Once treated the wood
returns to its natural original appearance.
Oxalic Acid-Based Products:
Certain wood species such as cedar and redwood contain natural resins
known as tannins. These are water soluble materials which are reddish brown
in color. Water can extract the resins from within the wood and deposit them
on the surface, leaving brown or black discolorations. Tannins can also react
with iron present in fasteners or nails. Resulting in blue-black stains. Neither
chlorine bleaches nor oxygen bleaches are effective against tannin stains or
iron stains. Oxalic acid, on the other hand, solubilizes tannins and iron stains
and renders them colorless. Thus it is the material of choice for use on
redwood or cedar. However, oxalic acid is not as effective against mildew. For
this reason some homeowners and contractors will treat redwood and cedar
with a sodium percarbonate or chlorine-based cleaner and follow it up with an
oxalic acid-based product if tannin staining is apparent. Concentrating oxalic
acid is toxic and should be handled and used with care.
Causes of Discoloration:
There are a number of sources of discoloration of wood decks. These include:
1) dirt and other foreign materials such as tree sap, bird droppings, grease,
2) fungal discolorations from mildew, mold, decay and sapstain growth
3) algae, moss and lichen growth
4) nail and other iron stains
5) tannins and other extractives from the wood
6) greying of the wood due to surface decomposition by sunlight and moisture
7) fading/decomposition of weathered coatings
Some of these discolorations are chemical in nature; others biological. All
require some effort on the part of the homeowner or contractor for removal
and all should be removed prior to refinishing.
In addition to being unsightly, these discolorations and the agents that cause
them can significantly interfere with the performance of any subsequently
applied coatings. Thus their removal is important from a performance as well
as an aesthetic standpoint.
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Water-repellent (WR) preservatives of some types are formulated with
non-drying oils. These oils penetrate the wood to protect it against
degradation. The deck surface may remain oily until the finish absorbs, which
may take several days.
Advantages of Water-Repellent Preservatives
Retard decay in above-ground applications
Decrease raised grain, checking, warping, and splitting
Inhibit mildew growth on both painted and unpainted wood
Retard iron stain
Stop extractive bleed
Improve paint adhesion
A Water Repellent Preservative (WRP) is an effective finish for a fully exposed
deck. Although the deck will need to be refinished frequently, there is no need
for laborious surface preparation, as is required by film-forming finishes.
Annual refinishing can be done quickly using a brush, roller, or pad. Brush
application works the finish into the wood better than do the other methods.
The finish should be applied liberally to decay-prone areas around fasteners
Pretreatments for Painted Wood
New Wood--Water repellent and WRP formulations for use as pretreatments
for paint have less wax or other water repellents compared with those
formulated for use without paint. When used as a pretreatment before
painting, a WRP can be applied in the same way as when used as a natural
finish. Freshly treated wood must be allowed to dry. If the treatment is applied
with a brush, allow 2 days of drying in warm weather before painting. If the
wood is dipped for 10 or more seconds, 1 week of drying is necessary before
painting. If enough time is not allowed for most of the solvent to dry from the
wood and for the wax to absorb, the paint applied over the treated wood may
not cure or bond properly. Open joints, such as in siding, miliwork, and facia,
should be caulked after treating with a WR or WRP but before priming.
Refinishing--When applying a WR or WRP to previously painted wood, loose
paint must be removed; the WR or WRP should be brushed into the joints and
unpainted areas. Remove excess WRP from the painted surfaces with a rag.
Allow 3 days of drying in warm weather before repainting.
Removal of Mold and Mildew
If mildew is present, pretreat the wood with a commercial cleaner or a chlorine
bleach-water solution. Allow the wood to dry for 1 or 2 days before refinishing.
Removal of Mildew
Commercially available wood cleaners work quite effectively to remove mildew
and other stains on wood. A mildew cleaner can also be made by dissolving 1
part liquid bleach and some powdered detergent in 2 to 4 parts water.
1/3 cup household detergent
1 quart (5%) sodium hypochlorite (household bleach)
3 quarts warm water
[Note: 1 cup =0.2 liter; 1 quart =0.9 liter]
CAUTION: Do not use a detergent that contains ammonia; ammonia reacts
with bleach to form a poisonous gas. Many liquid detergents may contain other
additives that react with chlorine- containing bleach.
The service life of WRs and WRPs is about 1 year on exposed wood surfaces.
However, WRPs are extremely easy to reapply to some structures, such as
decks. Water repellents and WRPs absorb readily into the end-grain of lumber
and can stop water absorption for many years.
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